Low-temperature heating element



June 17, 1947. a DRIVER 2,422,477

LOW TEMPERATURE HEATING ELEMENT Filed Nov. 1, 1944 IN VEN TOR. W716 ur8.. fink-er Patented June 17, 1947 UNITED STATES PATENT oFi-"lcsLOW-TEMPERATURE HEATING ELEMENT Wilbur B. Driver, East Orange, N. 1.,assignor to Wilbur B; Driver Company, Newark, N. 1., acorporation of NewJersey ApplicatlonNovember 1, 1944, Serial No. 561,391

3 Claims. 1

This invention relates to electrical resistance heating elements andmore particularly to such heating elements adapted for low temperatureheating ranges approximating 100 to 300 F., and has for its object theprovision of a heating element that is characterized by high flexibilityand high endurance limit.

Another object is to provide a flexible heating element for use at"relatively low heating temperatures approximating 300 F., that ischaracterized by having a high endurance limit and by having a crystalstructure resistant to alteration with consequent change in electricalcharacteristics as a result of flexing.

Still another object is to provide a flexible electrical resistanceheating element for use in electrically heated flying suits, blankets,heating pads, and the like articles of manufacture.

Gther objects and advantages will be apparent as the invention ismorefully hereinafter disclosed.

In accordance with these objects, I have discovered thatberyllium-copper alloys of the cold workable-precipitation hardenabletype in the solid solution or pure alpha phase, when in small section orsmall diameter, have the unexpected property of flexing withoutsubstantial work hardening at relatively low temperatures below about300 F. down to atmospheric temperatures and that this property alongwith a favorable electrical resistance adapts these alloys forutilization as high endurance flexible heating elements in such articlesof manufacture as elec trically heated flying suits, blankets, heatingpads, and the like, to impart to said articles a service life that isgreatly in excess of such articles heated with electrical resistanceelements comprised of any other type of alloy heretofore employed.

By the term beryllium-copper alloys of the cold workable-precipitationhardenable type" is meant that group of alloys heretofore known in theart containing from .30 to 3.0% beryllium and from small fractionalpercentages up to 3% of other metals and metalloids which arecharacterized by being capable of being converted by a high temperatureheat-treatment into a solid solution alloy which, after rapid cooling toatmospheric temperatures, is capable of being heat-treated at a lowertemperature to precipitation harden. Many diilerent compositions ofberyllium-copper alloys are known that fall within this broad class ofalloys, the particular amount of beryllium in the alloys varyingsomewhat with respect to the kind and amount of associated metal andmetalloid present. The associated metals and metalloids generallypresent in such alloys include Fe, Co, Ni, Si, Ag, P.

' referred to as the 2% Be alloy, the amount of Co or Ni present thereinbeing so low as to have a negligible effect upon the expected phasechange a properties of the binary beryllium-copper alloy of the samecomposition.

Such 2% beryllium-copper alloys after suitable hot working followingcasting, in accordance with known prior art methods and finalheattreating at temperatures within the range 1400- 1500 F., areconverted into the cold workable solid solution phase and by suitablecold working and heat-treating methods also known in the art suchmaterial ultimately may' be mechanically deformed into wire, sheet orstrip, of the desired diameter or thickness and by a final heat-treat-.

ment at a temperature within the range 1400- 1500 F. followed by rapidcooling may be converted interiorly into the solid solution phase forprecipitation hardening heat-treatment Within the temperature range 350to 550 F.

Heretofore in the art it has been recognized that solid solution alloysof the type obtained in the 2% Be beryllium-copper alloys, afterheattreatment at 14001500 F. and quenching, are essentially unstablealloys and that the phase changes that occur in such alloys aretime-temperature reactions which normally are accelerated by coldworking strains. Beryllium-copper alloys in this condition, for example,have been found to be relatively stable at atmospheric temperatures andstable against phase changes at temperatures below about 300 F., but oncold working have been found to strain harden so rapidly that a percentreduction in area approximating is about the maximum that may be appliedthereto between solution-anneals at 1400- 1500 F. to reconvert the coldworked metal to the unstrained solidsolution phase. It is recognizedgenerally that with increase in strain hardening the temperature ofprecipitation hardening decreases until at about 60% reductionin areaprecipitation hardening normally occurs in the cold worked metal at theusual temperatures reached during cold working, thereby hardening themetal sufficient to require solution-annealing before further coldworking may be practiced.

However, I have discovered that at temperatures within the rangeatmospheric to about 300 F., the 2% Be beryllium-copper alloys flex withlittle or no strain hardening, with the result that the normalresistance of the alloy to phase changes at temperatures below about 300F. is not altered by such flexing so that a relatively long seryice lifeunder variable flexing conditions may be obtained from such alloys.

Accordingly, by appropriate selection of wire diameter and length withrespect to its electrical resistance, flexible heating elementscomprised of such beryllium-copper alloys in the solid solution phase,may be formed for use with any given electrical load which arecharacterized predominately by a long life under variable flexingconditions along with a substantially constant electrical resistancecharacteristic.

One specific example of such heating element is illustrated in theattached drawings, wherein- Fig. 1 shows a single conductor comprised ofthe preferred alloy of the present invention in its heat-treatedcondition;

Fig. 2 shows a stranded conductor comprised of a plurality of theconductors of Fig. 1;

Fig. 3 shows the completed heating element of the present invention; and

Fig. 4 is a sectional view along plane 4-4 of Fig. 3.

As a specific example of the preferred alloy composition for theconductor a of Fig. 1 of the present invention, but not as a limitationof the same, a beryllium-copper alloy containing Be 1.85%, Co 25% andbalance Cu, has been found most suitable for the purposes of the presentinvention. This alloy in its solution-annealed condition normally has anelectrical resistance of about 60 ohms per circular mil foot at a wiresize of 003-.004 inch diameter.

Experimental tests have shown that this wire product withstands flexingin opposite directions alternately over a bend approximating a 90 angleat a rate of 60 cycles per minute, such alternate bends per minute for alength of time approximating 125 hours, or a total of 500,000 bendsbefore fracture with less than change in resistance. Th closest totalnumber of bends by any other wire heretofore proposed for such serviceuse under comparable test conditions was 25,00050,000 bends.

Electrical resistant tests on the beryllium-copper wire showed novariation or change in electrical resistance for 125 hours andthereafter only a small change up until shortly before the wire failedin the test in contrast to a relatively large and continuouslyincreasing change in electrical resistance in all other such conductorwires subjected to the same test.

Referring to the drawings, a flexible electrical resistance element forheating devices of the type described, is preferably formed of aplurality of wires A of about the above diameter (.003 to .004 inch)twisted together helically to form an electrically conductive strand ofthe desired electrical resistance, which strand is helically wound upona, flexible glass wool or flber core B of small diameter and held inhelically wound position thereon by means of a flexible cover sheath Ccomprised of non-conductive material such as a vinyl resin (aecu-polymerization product of vinyl chloride and vinyl acetate). In sucha construction of the heating element the flexibility of the helicalwinding of the flexible strand of wire on the flexible core normallytends to lower the flexing angle on the strand, increasing thereby thenormal life expectancy of the wire many times over that established bythe above noted test on the individual wire before stranding.

As specific examples of this construction, a strand B comprised of 10wires (.003" diameter) has an electrical resistance of about 1.3 ohmsper linear foot, whereas a strand B comprised of only 6 wires (.003"diameter) has an electrical resistance of 2.3 ohms per linear foot. Astrand comprised ofl2 wires (.003" diameter) has an electricalresistance of 12 ohms per linear foot. The spacing between the turns ofthe helically wound strand may be varied widely without essentialdeparture from the present invention as one skilled in the art willrecognize.

It is believed apparent to those skilled in the art of beryllium-copperalloys that many of the alloys may evidence flexing without substantialwork hardening at temperatures somewhat higher or somewhat lower than300 F. due to the retardant and accelerating eflects of the alloyconstituents present therein on the phase changes and ratio thereof.Cobalt, for example, lowers materially the heat-treating temperatures ofthe binary alloy, whereas nickel does not lower such heat-treatingtemperature to the same extent.

Having herelnabove described the present invention generically andspecifically, it is believed apparent to any one skilled in the art thatthe same may be widely varied without essential departure therefrom andall such modifications and departures therefrom ar contemplated as mayfall within the scope of the following claims.

What I claim is:

1. A flexible electrical resistance element for low temperature serviceuse below about 300 F., said element consisting of at least one smalldiameter conductor composed of a berylliumcopper alloy of the coldworkable-precipitation hardenable type, said alloy being in thesolutionannealed condition.

2. A flexible electrical resistance element for low temperature serviceuse below about 300 F., said heating element consisting of a pluralityof small diameter conductors helically twisted together to form astrand, each said conductor consisting of a beryllium-copper alloy ofthe cold workable-precipitation hardenable type in the solution-annealedcondition.

3. A flexible heating element for electrically heated flying suits,blankets, pads and the like, said heating element. consisting of aplurality of small diameter conductors hellcally twisted together toform a strand, said strand being helically wound on a flexibleelectrically non-conductive core and covered with an electricallynon-conductive sheath, said conductor consisting of a beryllium-copperalloy of the cold workable-precipitation hardenable type in thesolution-annealed condition.

WILBUR B. DRIVER.

REFERENCES CITED The following references are of record in th( flle ofthis patent:

UNITED STATES PATENTS Number Name Date 729,171 Herrgott May 26, 19031,170,811 Hay, et a1. Feb. 8, 1916 1,847,929 Dahl Mar. 1, 1932 1,959,154Bremer May 15, 1934 OTHER REFERENCES Donachies, The Role of BerylliumCopper as an Engineering Material," Sept. 13, 1944, 6 sheets.

A. S. F. M., Metals Handbook 1939. (See Div. 3, p, 1344.)

A. S. F. M., Age Hardenin of Metals" (pages 118-122), published by TheSociety for Metals,

Oct. 2'7, 1939. (Copy in Div. 3.)

